Composite film and preparation method and application thereof

Abstract

This invention provides a composite film, its preparation method, and its application, relating to the field of polymer composite packaging materials technology. The raw materials for the composite film include modified nanocellulose and polyphenol extract in a mass ratio of 2-18:1. The modified nanocellulose is obtained by oxidative modification and nano-sizing of cellulose, and its surface carboxyl group content is 0.5-1.8 mmol / g. This invention improves the tensile strength of the composite film and also exhibits inhibitory effects on common foodborne pathogens (Escherichia coli, Staphylococcus aureus), possessing antibacterial and antioxidant capabilities. Furthermore, it simultaneously achieves high optical transparency, high active ingredient loading, efficient UV shielding, and low migration, making it particularly suitable for food active packaging with high requirements for visual appearance and safety, and easy for industrial production.

Description

Technical Field

[0001] This invention relates to the field of polymer composite packaging materials technology, and in particular to a composite film, its preparation method and application. Background Technology

[0002] With the continuous advancement of production levels, people's requirements for food packaging materials are no longer limited to excellent antibacterial properties, but also include environmental friendliness and biodegradability. Currently, petroleum-based plastic films such as polyethylene (PE) and polypropylene (PP), which are widely used in food packaging, are non-degradable and lack bioactive functions, thus failing to meet the demand for biodegradability.

[0003] Nanocellulose (CNF) has gained increasing attention in the packaging field in recent years due to its wide availability, biodegradability, and good film-forming properties. Pure nanocellulose films lack antibacterial and antioxidant functions, therefore they often need to be compounded with other functional components (such as polyphenols) to improve their overall performance. However, CNF-polyphenol composite films formed by introducing polyphenols still have the following problems: (1) The introduction of active ingredients such as polyphenols will cause the membrane to become cloudy, which will reduce the visible light transmittance of CNF-polyphenol composite membrane and reduce optical transparency. However, in the field of high-end food packaging, the high optical transparency of packaging materials is crucial for displaying the color of the food itself and attracting consumers. (2) The binding force between polyphenols and the matrix in CNF-polyphenol composite film is weak, resulting in decreased mechanical properties. When in contact with food, the active ingredients are prone to migration, leading to a high migration rate, which poses a potential food safety risk and causes the preservation function to decay rapidly. (3) In order to achieve effective UV blocking to delay food photo-oxidation and extend shelf life, inorganic UV absorbers (such as TiO2, ZnO) or a large amount of dark active ingredients are often added, which leads to a significant reduction in visible light transmittance.

[0004] In summary, existing CNF-polyphenol composite membranes cannot simultaneously achieve the characteristics of high optical transparency, high active ingredient loading, efficient UV shielding, and low migration.

[0005] Therefore, this invention is proposed. Summary of the Invention

[0006] This invention provides a composite membrane, its preparation method, and its application. The composite membrane not only has excellent mechanical properties, long-lasting antibacterial and antioxidant effects, and degradability, but also simultaneously possesses high optical transparency, high active ingredient loading, efficient ultraviolet shielding, and low migration.

[0007] In a first aspect, the present invention provides a composite membrane comprising: modified nanocellulose and polyphenol extract in a mass ratio of 2-18:1; wherein the modified nanocellulose is obtained by oxidative modification and nano-sizing of cellulose, and the carboxyl content on its surface is 0.5-1.8 mmol / g.

[0008] The composite membrane of this invention uses modified nanocellulose with a surface carboxyl content of 0.5-1.8 mmol / g as a substrate. This modified nanocellulose interacts strongly with the phenolic hydroxyl groups of polyphenols (e.g., hydrogen bonds, ester bonds), effectively anchoring the active ingredients at the molecular level, achieving excellent antibacterial and antioxidant functions, and improving the tensile strength of the composite membrane. Furthermore, this invention has found that when modified nanofibers with a surface carboxyl content within the above range are used as raw materials, the composite membrane, while maintaining a high loading of active ingredients (polyphenols), can effectively improve its visible light transmittance (visible light transmittance ≥85%) and reduce the total phenol migration (total phenol migration ≤2.5 µg / dm³). 2 This technology ensures that the migration of its active ingredients is far below the limits set by relevant domestic and international food safety standards (such as GB 9685-2016 and GB 31604.1-2015). It also improves its UV blocking effect, achieving a UV blocking rate of ≥95%. This solves the problem that existing CNF-polyphenol composite films cannot simultaneously achieve high optical transparency, high active ingredient loading, efficient UV shielding, and low migration.

[0009] The mass ratio of modified nanocellulose to polyphenol extract is one of the key factors affecting the overall performance of the composite membrane. If the amount of polyphenol extract is too high, although it can improve UV blocking and antibacterial properties, excessive polyphenol molecules are prone to self-aggregation in the membrane matrix, forming independent polyphenol-rich microregions. This microscale phase separation disrupts the homogeneity of the membrane matrix, leading to increased optical haze and a significant decrease in visible light transmittance. Furthermore, since this excess polyphenol exists in the membrane as physical aggregates and fails to form an effective interfacial bond with the modified nanocellulose, it is prone to burst release upon contact with food, resulting in increased total phenol migration. Conversely, if the amount of polyphenol extract is too low, its active ingredients in the composite membrane are insufficient, failing to form a continuous and effective UV absorption layer, making it difficult to achieve a UV blocking rate of 95%, while also significantly weakening antibacterial and antioxidant functions. Therefore, by controlling the mass ratio of modified nanocellulose to polyphenol extract within the range of 2-18:1, it is possible to ensure that the composite film has high transparency while improving the UV blocking rate and reducing the migration amount, thus achieving a simultaneous improvement in UV shielding and anti-migration performance.

[0010] Preferably, the carboxyl content of the surface is 0.8-1.5 mmol / g, and the mass ratio of the modified nanocellulose to the polyphenol extract is 4-12:1.

[0011] Further, the step of oxidative modification of the cellulose includes: placing the cellulose in an oxidation system, reacting it for 1-2 h at a pH of 6-8 and a temperature of 0-10 ℃, then adjusting the pH to 9.5-10.5, and raising the temperature to 20-30 ℃ to continue the reaction for 3-8 h.

[0012] In the above scheme, a specific oxidation modification method is employed. First, in the first step reaction at a low temperature (0-10℃) and near neutral (pH 6-8), the oxidant preferentially and uniformly acts on the active hydroxyl groups on the amorphous regions of cellulose and the surface of microfibers, achieving uniform anchoring to achieve pre-oxidation and effectively avoiding cellulose degradation caused by localized over-oxidation. Subsequently, in the second step reaction at a higher temperature (20-30℃) and alkaline (pH 9.5-10.5), deep oxidation is carried out, efficiently converting the pre-oxidized sites into carboxyl groups, thereby forming a uniformly distributed and moderately dense layer of carboxyl functional groups on the surface of cellulose microfibers. Compared with the conventional one-step oxidation method, the two-step oxidation method of this invention significantly reduces the degradation of the cellulose bulk structure, maintains the high aspect ratio and intact crystalline structure of the nanofibers, enabling them to form a denser and more uniform network structure during subsequent film formation. By employing the specific oxidative modification method of this invention, the carboxyl groups uniformly distributed in the modified nanocellulose can form a more ordered and stronger hydrogen bond or ester bond crosslinking network with the phenolic hydroxyl groups of polyphenols, thereby achieving molecular-level anchoring of polyphenol molecules. This effectively inhibits the migration of polyphenols while ensuring high UV blocking rate, and also effectively improves the mechanical properties and visible light transmittance of the composite film.

[0013] Preferably, the oxidation system comprises TEMPO, NaBr, and NaClO in a mass ratio of 1:(5-10):(50-100), wherein the amount of NaClO relative to the available chlorine in the cellulose is 4-10 mmol / g. Controlling the amount of available chlorine relative to cellulose within the above range helps to precisely regulate the carboxyl group content on the surface of nanocellulose to 0.5-1.8 mmol / g.

[0014] Preferably, the effective chlorine concentration in the NaClO is 10%-15% (w / w).

[0015] Furthermore, the nano-processing step includes: washing the oxidized product first, and then sequentially performing ultrasonic pretreatment and high-pressure microfluidic treatment.

[0016] Furthermore, the parameters of the ultrasound pretreatment include: power of 200-400 W and time of 15-30 min.

[0017] Furthermore, the parameters of the high-pressure microjets treatment include: a pressure of 800-1200 bar, and 2-4 cycles.

[0018] Furthermore, the cellulose is selected from one or more of softwood pulp cellulose, hardwood pulp cellulose, bamboo pulp cellulose, and bacterial cellulose.

[0019] More preferably, the cellulose is bacterial cellulose.

[0020] This invention has revealed that the type of cellulose affects the mechanical, optical, UV blocking, and total phenol migration properties of the prepared composite membrane. When using the four types of cellulose as raw materials, the resulting composite membranes all possess characteristics of high optical transparency, high loading of active ingredients, efficient UV shielding, and low migration, with bacterial cellulose exhibiting the best overall performance.

[0021] In some optional embodiments of the present invention, the degree of polymerization of the bamboo pulp cellulose is 500-600, the α-cellulose content is ≥95%, and it is preferably bleached bamboo pulp board.

[0022] In some optional embodiments of the present invention, the degree of polymerization of the softwood pulp cellulose is 800-1000, the α-cellulose content is ≥90%, and it is preferably bleached softwood sulfate pulp.

[0023] In some optional embodiments of the present invention, the degree of polymerization of the hardwood pulp cellulose is about 600-800, the α-cellulose content is ≥92%, and it is preferably bleached hardwood sulfate pulp.

[0024] In some optional embodiments of the present invention, the degree of polymerization of the bacterial cellulose is 2000-3000, and the purity is ≥99%. Preferably, it is prepared by fermentation, and the preparation method includes: activating *Gluconacetobacter xylinus*, inoculating it into HS medium at an inoculum of 3%-8% (v / v), statically culturing it at 28-30℃ for 5-7 days to form a cellulose membrane, treating the cellulose membrane with 0.1-0.5 mol / L NaOH solution at 80-90℃ for 1-2 hours to remove bacterial cells and culture medium residues, then washing it with deionized water until neutral to obtain a pure bacterial cellulose wet membrane, and freeze-drying it to obtain the bacterial cellulose.

[0025] Furthermore, the HS culture medium is composed of 2% glucose (w / v), 0.5% peptone (w / v), 0.5% yeast extract (w / v), 0.27% disodium hydrogen phosphate (w / v), 0.115% citric acid (w / v), and pH 5.0-6.0.

[0026] Furthermore, the total phenol content in the polyphenol extract is not less than 50 mg GAE / g. When the total phenol content is less than 50 mg GAE / g, it will not only reduce the antibacterial and antioxidant properties of the composite membrane, but also make it difficult to form an effective ultraviolet absorption network in the membrane, resulting in a decrease in ultraviolet blocking rate; at the same time, a large number of inactive impurities in the extract will affect the uniformity of the membrane matrix and reduce optical transparency.

[0027] Preferably, the total phenol content in the polyphenol extract is 50-200 mg GAE / g. Within this range, the functionality can be guaranteed without affecting the light transmittance and structural density of the membrane due to excessive impurities.

[0028] Preferably, the polyphenol extract is obtained from one or more raw materials selected from rose petals, grape seeds, grape skins, and tea leaves.

[0029] In some optional embodiments of the present invention, the extraction method of the polyphenol extract includes: using at least one of rose, grape seed, grape skin, and tea as raw materials, drying and pulverizing the raw materials, and passing them through a 20-60 mesh sieve. Adding an ethanol-water solution at a material-to-liquid ratio of 1g:(10-30)mL for ultrasonic extraction, combining the extracts, filtering, concentrating, and drying to obtain the polyphenol extract.

[0030] Preferably, the parameters for ultrasonic extraction include: temperature of 40-60℃, time of 30-90 min, and extraction 2-3 times.

[0031] Preferably, the volume fraction of the ethanol aqueous solution is 50%-80%.

[0032] Preferably, the concentration temperature is 40-50℃, and the volume is concentrated under reduced pressure to 1 / 5-1 / 10 of the original volume.

[0033] Furthermore, the modified nanocellulose has a cross-sectional diameter of 5-30 nm and a length of 200-600 nm. Specifically, when modified nanocellulose is prepared from softwood pulp cellulose, hardwood pulp cellulose, bamboo pulp cellulose, and bacterial cellulose, the cross-sectional diameter of the modified nanocellulose is mainly distributed in the range of 5-25 nm, and the length is mainly distributed in the range of 200-600 nm. When modified nanocellulose is prepared from bacterial cellulose, due to the unique three-dimensional network structure and high degree of polymerization of bacterial cellulose, the diameter of the modified nanocellulose obtained after modification is slightly larger (10-40 nm), and the length can reach 1-5 μm.

[0034] Furthermore, the degree of polymerization (reflecting molecular weight) of the modified nanocellulose is ≥200, preferably 300-2500, meeting the requirements of T / CCEIA 0002-2024 standard (degree of polymerization ≥100). The degree of polymerization varies depending on the source of the cellulose raw material used in the preparation of the modified nanocellulose. Specifically: when using softwood pulp or hardwood pulp cellulose as raw material, the degree of polymerization of the modified nanocellulose is preferably 300-1000; when using bacterial cellulose as raw material, the degree of polymerization of the modified nanocellulose is preferably 1000-2500.

[0035] Furthermore, the raw material also includes a plasticizer, wherein the plasticizer comprises 0.5-5% by mass in the raw material; And / or, the raw material further includes a crosslinking agent, wherein the crosslinking agent comprises 0.1-2% by mass in the raw material.

[0036] Preferably, the plasticizer includes one or more of glycerol, sorbitol, and polyethylene glycol-400.

[0037] Preferably, the crosslinking agent includes one or more of glutaraldehyde and epichlorohydrin.

[0038] Further, by mass percentage, the raw material comprises the following components: 60-90% modified nanocellulose, 5-30% polyphenol extract, 0.5-5% plasticizer, and 0.1-2% crosslinking agent.

[0039] In some optional embodiments of the present invention, the raw materials, by mass percentage, comprise the following components: 60-90% modified nanocellulose, 5-30% rose polyphenol extract, 0.5-5% glycerol, and 0.1-2% glutaraldehyde. The composite membrane prepared by combining rose polyphenol extract with modified nanocellulose, and using glycerol as a plasticizer and glutaraldehyde as a crosslinking agent in this system, has better overall performance.

[0040] Furthermore, the composite membrane has at least one of the following characteristics: a. Transmittance at 600 nm wavelength ≥ 85%; b. UV blocking rate at 400 nm wavelength ≥95%; c. In a simulated migration test at 40℃ for 10 days, the total phenol migration amount was ≤2.5 µg / dm³. 2 .

[0041] Preferably, the composite membrane also has at least one of the following characteristics: d and e: d. Thickness is 20-80 μm; e. Tensile strength ≥ 70 MPa.

[0042] In a second aspect, the present invention provides a method for preparing the above-mentioned composite film, comprising the following steps: mixing the modified nanocellulose, the polyphenol extract, the plasticizer and the crosslinking agent evenly, subjecting the mixture to ultrasonic treatment to obtain a film-forming liquid; then casting the film-forming liquid onto a substrate to form a film, and drying it.

[0043] Preferably, the modified nanocellulose is provided in the form of a dispersion, wherein the mass of the modified nanocellulose accounts for 0.5-2% of the mass of the dispersion; more preferably, the mass of the modified nanocellulose accounts for 1% of the mass of the dispersion.

[0044] Preferably, the polyphenol extract is provided in the form of a solution with a mass concentration of 0.5-5%.

[0045] Preferably, the parameters of the ultrasonic treatment include: power of 200-400 W and time of 10-60 min.

[0046] In one optional embodiment of the present invention, the method for preparing the composite membrane includes the following steps: S1. Preparation of modified nanocellulose dispersion: The cellulose is placed in the oxidation system for the oxidation modification treatment, so that the carboxyl content on the surface is precisely controlled within the range of 0.5-1.8 mmol / g. After the reaction is completed, the product is washed until neutral and then subjected to the nano-sizing treatment to obtain a modified nanocellulose dispersion with uniform dispersion, intact fiber morphology and uniform distribution of carboxyl groups on the surface. S2. Raw material blending: The modified nanocellulose dispersion, polyphenol extract aqueous solution, plasticizer and crosslinking agent are mixed in proportion and stirred evenly at room temperature to obtain a blend; the preferred stirring time is 30-120 minutes. S3. Ultrasonic treatment: The blend is subjected to ultrasonic treatment to obtain a stable film-forming liquid; S4. Casting and drying: Cast the film-forming solution onto the substrate, dry it at 40-60 ℃ for 4-12 h, peel off the film, and obtain the composite film.

[0047] A third aspect of the present invention provides the application of the above-described composite film or the composite film prepared by the above-described method in food packaging and food preservation.

[0048] Preferably, the composite film is used in the preservation of fresh shrimp.

[0049] The beneficial effects of the composite membrane, its preparation method, and its application provided by this invention include at least the following: (1) The composite film preparation process of the present invention is simple, requires no complicated equipment, is safe and reliable to use, and the substrate is a completely biodegradable modified nanocellulose, which is environmentally friendly; it has good compatibility with industrial casting production lines, the raw material cost is controllable (≤15 USD / kg), has the potential for large-scale production, and is easy to industrialize. (2) The composite film of the present invention uses modified nanocellulose with a surface carboxyl content of 0.5-1.8 mmol / g as a substrate. It interacts strongly with the phenolic hydroxyl groups of polyphenols, which improves the tensile strength of the composite film. It also shows an inhibitory effect on common foodborne pathogens (Escherichia coli, Staphylococcus aureus), especially the inhibitory effect on Escherichia coli, and has antibacterial and antioxidant capabilities. Moreover, it can simultaneously achieve the characteristics of high optical transparency, high active ingredient loading, high efficiency UV shielding and low migration, which is particularly suitable for food active packaging with high requirements for visual appearance and safety. It solves the problem that existing CNF-polyphenol composite films cannot simultaneously achieve high optical transparency, high active ingredient loading, high efficiency UV shielding and low migration. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0051] Figure 1 This is an appearance diagram of the composite membrane prepared in Example 1.

[0052] Figure 2 This is a SEM image of the composite membrane prepared in Example 1.

[0053] Figure 3 This is a SEM image of the composite membranes prepared by different mass ratios of modified nanocellulose and polyphenol extract in Experiment Example 1.

[0054] Figure 4 The image shows the FTIR spectra of composite membranes prepared by different mass ratios of modified nanocellulose and polyphenol extract in Experiment Example 1.

[0055] Figure 5 This is an image of the antibacterial zone of composite membranes prepared by different mass ratios of modified nanocellulose and polyphenol extract in Experiment Example 1.

[0056] Figure 6 The images show actual photos and color indicators of the composite film from Example 1 used for preserving fresh shrimp at 4°C.

[0057] Figure 7This is a graph showing the pH change over storage days when the composite membrane of Example 1 is used for fresh shrimp preservation. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0059] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0060] The raw material information for the following examples and comparative examples is as follows: Bamboo pulp cellulose: Purchased from Fuzhou Bamboo Fiber Materials Technology Co., Ltd., it is bleached bamboo pulp board with a degree of polymerization of 500-600 and an α-cellulose content of ≥95%.

[0061] Softwood pulp cellulose: purchased from Suzhou Cellutech Chemical Co., Ltd., is bleached softwood sulfate pulp with a degree of polymerization of 800-1000 and an α-cellulose content of ≥90%.

[0062] Hardwood pulp cellulose: purchased from Suzhou Cellutech Chemical Co., Ltd., is bleached hardwood sulfate pulp with a degree of polymerization of 600-800 and an α-cellulose content of ≥92%.

[0063] Bacterial cellulose: Prepared by fermentation. The specific preparation method includes: fermenting *Acetobacter xylodis* (…). Gluconacetobacter xylinus After activation with CGMCC 1.1812, the bacterial cellulose was inoculated into HS medium (2% glucose (w / v), 0.5% peptone, 0.5% yeast extract, 0.27% disodium hydrogen phosphate, 0.115% citric acid, pH 5.0-6.0) and statically cultured at 30℃ for 7 days to form a cellulose membrane. The membrane was then treated with 0.3 mol / L NaOH solution at 85℃ for 1-2 hours to remove bacterial cells and residual culture medium. It was then washed with deionized water until neutral to obtain a pure bacterial cellulose wet membrane. After freeze-drying, bacterial cellulose with a degree of polymerization of 2500 and a purity ≥99% was obtained. The HS medium consisted of 2% glucose (w / v), 0.5% peptone (w / v), 0.5% yeast extract (w / v), 0.27% disodium hydrogen phosphate (w / v), 0.115% citric acid (w / v), and pH 5.0-6.0.

[0064] The preparation method of rose polyphenol extract includes: drying and pulverizing rose petals, and passing them through a 40-mesh sieve; adding a 65% (v / v) ethanol aqueous solution to the dried rose petals at a material-to-liquid ratio of 1g:20mL, and extracting at 40℃ with ultrasonic assistance for 60min, repeating the extraction three times. The extracts are combined, filtered, concentrated under reduced pressure at 40℃ to 1 / 6 of the original volume, and then freeze-dried to obtain polyphenol extract powder.

[0065] The performance testing methods in the following examples, comparative examples, and experimental cases are as follows: Thickness: Measure at multiple points using a digital micrometer and take the average value.

[0066] Transmittance: Measured at a wavelength of 600 nm using a UV-Vis spectrophotometer.

[0067] Tensile strength: Tested using a universal testing machine in accordance with GB / T 1040.3-2006.

[0068] Inhibition zone diameter: The Kirby-Bauer method was used for the agar diffusion method, referring to the CLSI standard.

[0069] Total phenol content: Folin-Ciocalteu method, calculated as gallic acid.

[0070] Color difference: The CIE L, a, and b values ​​were measured using a colorimeter.

[0071] Fourier transform infrared spectroscopy (FTIR): KBr pellet method.

[0072] Scanning electron microscope (SEM): Observing the surface morphology after gold sputtering.

[0073] Carboxyl content: Determined according to GB / T 10338-2008, the specific steps are as follows: (1) Sample taking: Representative samples shall be taken in accordance with the provisions of GB / T 740.

[0074] (2) Moisture determination: The moisture content of the sample shall be determined in accordance with GB / T 462 and used to calculate the mass of the oven-dried sample.

[0075] (3) Sample dissociation: Weigh a sample equivalent to approximately 1.5 g of oven-dry weight. For chemical pulp, perform wet dissociation according to ISO 5263-1; for mechanical pulp, perform wet dissociation according to ISO 5263-2 or ISO 5263-3 to ensure adequate fiber dispersion.

[0076] (4) Preparation of filter cake: The dissociated fiber suspension is transferred to a glass filter with a filter plate, washed with deionized water and then filtered to form a uniform filter cake on the filter plate.

[0077] (5) Determination: Ion exchange, EDTA titration and result calculation shall be performed in accordance with the provisions of Chapters 6 and 7 of GB / T 10338-2008. The carboxyl content is expressed as mmol / g of oven-dry sample.

[0078] UV blocking rate: determined by UV-Vis spectrophotometer, the specific steps are as follows: (1) Sample preparation: Cut the composite film into rectangular strips of 30 mm × 10 mm, ensuring that the surface is flat, without wrinkles or bubbles. Each sample is measured in parallel 3 times.

[0079] (2) Instrument settings: Use a UV-Vis spectrophotometer (PerkinElmerLambda 35) equipped with an integrating sphere accessory, with the scanning wavelength range set to 200-800 nm, scanning speed 240 nm / min, and slit width 2 nm.

[0080] (3) Baseline calibration: Baseline calibration is performed using a blank cuvette as a reference.

[0081] (4) Transmittance measurement: Place the composite film sample in close contact with the incident light side of the cuvette and measure the transmittance (T, %) at different wavelengths. Record the transmittance values ​​at 200-400 nm and 550 nm.

[0082] (5) Calculation of UV blocking rate: The UV blocking rate is calculated according to the following formula: ; In the formula: A UV : Refers to the absorbance of the sample in the ultraviolet band (280-400 nm). A 总 : Refers to the absorbance of the sample across the entire wavelength range.

[0083] Total phenol migration: determined in accordance with GB 31604.46-2023 "National Food Safety Standard: Determination of Free Phenols and Migration in Food Contact Materials and Articles".

[0084] Example 1 This embodiment provides a composite membrane whose raw materials, by mass percentage, include the following components: 77.8% modified nanocellulose, 19.4% rose polyphenol extract, 2.3% glycerol, and 0.5% glutaraldehyde.

[0085] The modified nanocellulose is obtained by oxidative modification and nano-sizing of cellulose, with a surface carboxyl group content of 0.8 mmol / g, a diameter of 5-30 nm, and a length of 200-600 nm; the mass ratio of the modified nanocellulose to the polyphenol extract is 4:1. The total phenol content in the polyphenol extract is 120 mg GAE / g.

[0086] This embodiment also provides a method for preparing the above-mentioned composite membrane, including the following steps: S1. Preparation of modified nanocellulose dispersion: 5g of bamboo pulp cellulose was placed in 500 mL of oxidation system and reacted at pH 7 and temperature 5 ℃ for 2 h. Then, the pH was adjusted to 10, and the temperature was raised to 25 ℃ for another 6 h to carry out the oxidation modification treatment. After the reaction, the product was washed until neutral, then ultrasonically pretreated at 300 W for 25 min, and then subjected to high-pressure micro-jet treatment at 1000 bar for 3 cycles to achieve efficient nano-sizing, resulting in a modified nanocellulose dispersion with uniform dispersion, intact fiber morphology, and uniform surface carboxyl group distribution. The mass of the modified nanocellulose accounted for 1% of the mass of the modified nanocellulose dispersion. 100 mL of the dispersion was freeze-dried to obtain solid modified nanocellulose. The carboxyl group content on the surface of the modified nanocellulose was found to be 0.8 mmol / g. S2. Raw material blending: Take 100 g of the modified nanocellulose dispersion (containing 1 g of modified nanocellulose), 12.4 g of rose polyphenol extract aqueous solution (containing 0.25 g of rose polyphenol extract), 0.03 g of glycerol and 0.006 g of glutaraldehyde, mix them, and stir magnetically at room temperature for 60 min to obtain a blend. S3. Ultrasonic treatment: The blend is ultrasonically treated at a power of 300 W for 30 min to obtain a stable film-forming liquid. S4. Casting and Drying: The film-forming solution is cast onto a substrate and dried at 50 °C for 8 h. The film is then peeled off to obtain the composite film with a thickness of 42 μm. The appearance of the composite film is shown in the figure below. Figure 1 As shown, its SEM image is as follows: Figure 2 As shown, by Figure 1 and Figure 2 It can be seen that the membrane surface is dense and smooth, the modified nanocellulose forms a continuous network, the polyphenols are evenly distributed, there is no phase separation, and the interface bonding is good.

[0087] The raw materials for the oxidation system include: 0.05 g TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy radical), 0.5 g NaBr, and 2.5 mL of NaClO solution with an effective chlorine concentration of 11.5% (w / w). The effective chlorine content of NaClO relative to cellulose is 0.89 mmol / g. The preparation method is as follows: First, dissolve 0.05 g TEMPO and 0.5 g NaBr in 475 mL of deionized water and stir magnetically at room temperature until completely dissolved to obtain a mixed solution. Then, add 25 mL of NaClO solution with an effective chlorine concentration of 11.5% to this mixed solution and stir until homogeneous to obtain an oxidation system with a total volume of 500 mL.

[0088] Example 2 The difference between 2-1 to 2-3 of this embodiment and Example 1 lies in the different parameters of the oxidation modification step in step S1. The specific differences are as follows: Example 2-1 5g of bamboo pulp cellulose was placed in an oxidation system (same as in Example 1), and reacted for 2 hours at pH 6 and temperature 10°C. Then, the pH was adjusted to 9.5, and the temperature was raised to 20°C to continue the reaction for 8 hours. Subsequent steps were the same as in Example 1.

[0089] The modified nanocellulose prepared in Example 2-1 was found to have a carboxyl group content of 0.6 mmol / g, a cross-sectional diameter of 5-30 nm, and a length of 200-600 nm. The performance test results of the prepared composite membrane are shown in Table 1.

[0090] Example 2-2 5g of bamboo pulp cellulose was placed in an oxidation system (same as in Example 1), and reacted for 1.5h at pH 7 and 5°C. Then the pH was adjusted to 10, and the temperature was raised to 25°C to continue the reaction for 5.5h. Subsequent steps were the same as in Example 1.

[0091] The modified nanocellulose prepared in Examples 2-2 was found to have a carboxyl content of 0.9 mmol / g, a cross-sectional diameter of 5-30 nm, and a length of 200-600 nm. The performance test results of the prepared composite membrane are shown in Table 1.

[0092] Example 2-3 5g of bamboo pulp cellulose was placed in an oxidation system (same as in Example 1), and reacted for 1 hour at pH 8 and 0°C. Then, the pH was adjusted to 10.5, and the temperature was raised to 30°C to continue the reaction for 3 hours. Subsequent steps were the same as in Example 1.

[0093] The modified nanocellulose prepared in this embodiment had a carboxyl group content of 1.3 mmol / g, a cross-sectional diameter of 5-30 nm, and a length of 200-600 nm. The performance test results of the prepared composite membrane are shown in Table 1.

[0094] Example 3 The difference between this embodiment and Embodiment 1 is that the cellulose used in preparing the composite membrane is softwood pulp cellulose.

[0095] The modified nanocellulose prepared in this embodiment had a carboxyl content of 0.9 mmol / g, a cross-sectional diameter of 5-30 nm, and a length of 300-600 nm. The performance test results of the prepared composite membrane are shown in Table 1.

[0096] Example 4 The difference between this embodiment and Embodiment 1 is that the cellulose used in preparing the composite membrane is hardwood pulp cellulose.

[0097] The modified nanocellulose prepared in this embodiment had a carboxyl group content of 1.0 mmol / g, a cross-sectional diameter of 5-30 nm, and a length of 200-500 nm. The performance test results of the prepared composite membrane are shown in Table 1.

[0098] Example 5 The difference between this embodiment and Embodiment 1 is that the cellulose used in preparing the composite membrane is bacterial cellulose.

[0099] The modified nanocellulose prepared in this embodiment had a carboxyl group content of 1.2 mmol / g, a cross-sectional diameter of 10-40 nm, and a length of 1-5 μm, as determined by testing. The performance test results of the prepared composite membrane are shown in Table 1.

[0100] Example 6 The difference between this embodiment and Embodiment 1 is that in step S1 of preparing the composite membrane, the oxidized product is washed and then directly subjected to high-pressure microfluidic treatment without ultrasonic pretreatment. The composite membrane prepared in this embodiment exhibits visible particles on its surface and poor uniformity.

[0101] Example 7 The difference between this embodiment and Example 1 is that step S1 in preparing the composite membrane uses a one-step oxidation method for oxidation modification. Specifically, 5g of bamboo pulp cellulose is placed in a 500 mL oxidation system (the same as in Example 1) and reacted at pH 10 and temperature 25 ℃ for 8 h to carry out the oxidation modification treatment.

[0102] Comparative Example 1 This comparative example uses nanocellulose without oxidative modification to prepare a composite membrane. The preparation method differs from Example 1 in that, in step S1, 5g of bamboo pulp cellulose is placed in 500 mL of phosphate buffer solution with a pH of 10, and then subjected to the same ultrasonic pretreatment and high-pressure microfluidic treatment steps as in Example 1 to obtain a nanocellulose dispersion. Other steps are the same as in Example 1. The carboxyl content on the surface of the nanocellulose obtained in this comparative example is 0.08 mmol / g. The composite membrane prepared in this comparative example shows polyphenol particle aggregation and a tendency for phase separation.

[0103] Comparative Example 2 The composite membrane of this comparative example does not contain polyphenol extract in its raw materials. The difference between its preparation method and that of Example 1 is that no aqueous solution of rose polyphenol extract is added when the raw materials are blended in step S2.

[0104] The antibacterial properties, light transmittance, total phenol migration, ultraviolet blocking rate, and tensile strength of the composite films prepared in the examples and comparative examples were measured in this invention. The bacterial solution concentration used for the inhibition zone diameter test was 0.5% (approximately 10). 7 The concentration of CFU / mL was determined using the agar diffusion method, and the results are shown in Table 1. In the table, " / " indicates that no polyphenols were added in Comparative Example 2, therefore, there is no data on the total phenol migration.

[0105] As shown in Table 1, the composite films corresponding to Examples 1-7 can simultaneously achieve the characteristics of high optical transparency, high mechanical properties, high efficiency UV shielding, low migration, and high antibacterial properties, especially the composite films corresponding to Examples 1-5, which have the best overall performance.

[0106] Experimental Example 1 This experiment investigated the effect of the mass ratio of modified nanocellulose (named NC) to polyphenol extract (named PP) on the performance of composite membranes and determined the optimal ratio. The experimental method was as follows: Composite membranes were prepared according to the method in Example 1, with the amount of modified nanocellulose added remaining constant (1g), and the mass percentages of glycerol and crosslinking agent in the composite membrane raw materials kept the same as in Example 1. Based on Example 1, the amount of polyphenol extract added was adjusted to obtain composite membranes with different NC:PP mass ratios. The groups were set as follows: rBC-T1: NC; rBC-T5:NC:PP = 4:1 (PP addition amount 0.25 g, same as in Example 1); rBC-T 10 NC:PP = 3:1 (PP addition amount 0.33 g).

[0107] SEM images of the composite membranes prepared in the above groups are shown below. Figure 3 As shown, Figure 3 A small amount of agglomeration occurred between the fibers in the composite membranes corresponding to rBC-T5 and rBC-T10. With the increase of PP addition, the fiber shape arrangement of the composite membrane became more disordered, and the mesh diameter was smaller and denser. This indicates that cross-linking between the phenolic hydroxyl groups (Ar-OH) and -COOH in the rose polyphenol structure enhances the fiber network structure.

[0108] The FTIR spectra of the composite membranes prepared in the above groups are as follows: Figure 4 As shown in the figure, the addition of PP did not cause a significant shift in the position of the characteristic peak (1200-1300 cm⁻¹). -1 The nearby absorption peaks are due to the stretching vibrations of CO and C; 1550 cm⁻¹ -1 The absorption peak at 1026 cm⁻¹ is due to the skeletal vibration of the benzene ring; -1The absorption peaks around the left and right are due to the CH bending vibration on the aromatic ring, indicating that PP was successfully attached to the modified nanocellulose and bonded to the modified nanocellulose through non-covalent crosslinking.

[0109] The antibacterial zones of the composite films prepared in the above groups are as follows: Figure 5 As shown, the inhibition zone diameter around the rBC composite membrane was measured to evaluate its effectiveness against Escherichia coli (E. coli). coli , ATCC 8099) and Staphylococcus aureus (S. aureus The antibacterial effect of rBC composite membrane (ATCC6538) was investigated. Three different concentrations of bacterial solutions (2%, 0.5%, and 1%) were prepared to explore the antibacterial effect of the composite membrane. The results showed that the rBC composite membrane exhibited inhibitory effects on *Escherichia coli* and *Staphylococcus aureus* at concentrations of 0.5% and 0.01%, especially showing a significant inhibitory effect on *Escherichia coli*. Furthermore, with increasing PP addition, the diameter of the inhibition zone of the composite membrane increased, indicating that the increase in polyphenols could improve the antibacterial properties of the rBC composite membrane.

[0110] The colorimetric values ​​and thicknesses of each composite film group are shown in Table 2. Table 2. Effects of different NC and PP mass ratios on the color value and thickness of the composite film.

[0111] Table 2 shows that the rBC-T1 film has a high L value, while the a and b values ​​are low. After adding PP, the L value of the composite film decreases, while a and b increase, indicating a decrease in brightness and an increase in red and yellow hues. These results demonstrate that adjusting the mass ratio of modified nanocellulose to polyphenol extract can improve the optical properties of the composite film to meet different optical performance requirements.

[0112] Experiment Example 2 This experimental example follows the method described in Experiment 1 to prepare composite films with different NC:PP mass ratios. The effects of the mass ratio on the antibacterial properties, light transmittance, total phenol migration, UV blocking rate, and tensile strength of the composite films were investigated. The bacterial solution concentration used for the inhibition zone diameter test was 0.5% (approximately 10). 7 The concentration of CFU / mL was determined using the agar diffusion method, and the results are shown in Table 3. Table 3. Effect of different NC:PP mass ratios on composite membrane performance

[0113] Based on the above performance indicators, a mass ratio of modified nanocellulose to polyphenol extract controlled within the range of 2-18:1 can achieve excellent UV shielding (UV blocking rate of 88.6%-97.8%, with a maximum of ≥95% when the mass ratio is ≤8:1) while ensuring high optical transparency (transmittance ≥85%), and simultaneously keeping the total phenol migration within a safe range (≤2.5 µg / dm³). 2 While maintaining good mechanical properties and antibacterial activity, when the mass ratio of modified nanocellulose to polyphenol extract is >18:1 or <2:1, at least one key property of the composite film is significantly degraded, making it difficult to meet the comprehensive performance requirements of food active packaging.

[0114] Experimental Example 3 This experiment obtained modified nanocellulose with different surface carboxyl group contents by adjusting the amount of NaClO relative to the effective chlorine of cellulose during the oxidation modification process (as shown in Table 4). A composite membrane was then prepared according to the method in Example 1. The effects of the carboxyl group content on the surface of the modified nanocellulose on the antibacterial properties, transmittance, total phenol migration, UV blocking rate, and tensile strength of the composite membrane were studied. The bacterial solution concentration used for the inhibition zone diameter test was 0.5% (approximately 10). 7 The concentration of CFU / mL was determined using the agar diffusion method, and the results are shown in Table 4. Table 4 shows a significant nonlinear relationship between the carboxyl content on the surface of modified nanocellulose and the performance of the composite membrane: when the carboxyl content is below 0.5 mmol / g, the number of carboxyl groups is insufficient, resulting in weak interaction with polyphenols, leading to poor polyphenol anchoring (high migration) and a loose network structure (low transmittance and strength); when the carboxyl content is in the range of 0.5-1.8 mmol / g, the carboxyl groups form a moderate hydrogen / ester bond crosslinking network with the phenolic hydroxyl groups of polyphenols, and all properties reach an excellent level; however, when the carboxyl content is above 1.8 mmol / g, excessive oxidation leads to cellulose degradation, damage to the network structure, and a decrease in transmittance and tensile strength, while the improvement effect on UV blocking and migration tends to saturate. Therefore, controlling the carboxyl content on the surface of modified nanocellulose between 0.5-1.8 mmol / g can result in a composite membrane with superior overall performance.

[0115] Experiment Example 4 To further verify the preservation effect of the composite film prepared in this invention, the application of the composite film prepared in Example 1 in the preservation of fresh shrimp is provided, and its effect is compared with that of polyethylene (PE) preservation film. The experimental method is as follows: (I) Experimental Design Fresh shrimp (purchased from the market, with an initial total bacterial count ≤3 log CFU / g and TVB-N value ≤5 mg / 100g) were randomly divided into two groups: Experimental group: shrimp were wrapped in the composite membrane prepared in Example 1 (approximately 100g per group), named BC-T.

[0116] Control group: wrapped with ordinary commercially available polyethylene (PE) plastic wrap, named rBC-T5.

[0117] All samples were stored in a refrigerator at 4±1℃ and were sampled and tested on days 0, 3, 5, 7 and 10.

[0118] (II) Detection Indicators and Methods Sensory evaluation: Following GB / T 37062-2018 "Guidelines for Sensory Evaluation of Aquatic Products," fresh shrimp were comprehensively evaluated based on four dimensions: color, odor, elasticity, and mucus (out of 10). The scoring criteria are as follows: 9-10 points: Excellent (natural and glossy color, strong inherent odor, firm and elastic texture, no mucus); 7-8 points: Good (slightly darker color, weaker odor, relatively good elasticity, slightly sticky surface); 5-6 points: Medium (significantly darker color, slightly fishy smell, poor elasticity, small amount of mucus); 3-4 points: Poor (dull and dull color, obvious fishy smell, poor elasticity, excessive mucus); 1-2 points: Inferior (grayish-black color, putrid smell, no elasticity, excessive mucus). A comprehensive score <6 points was deemed sensorily unacceptable (end of shelf life). The evaluation was conducted independently by 12 trained evaluators.

[0119] Overall assessment (out of 10).

[0120] pH value: The shrimp meat homogenate was measured using a pH meter.

[0121] Color rendering effect of composite film: actual product image.

[0122] (III) Results and Analysis Actual images and color indicators of the composite film used for shrimp preservation at 4℃ are shown. Figure 6 As shown in the figure, the pH value changes with the number of storage days. Figure 7 As shown.

[0123] The results showed that, compared with PE film packaging, the polyphenol-nanocellulose composite film prepared by this invention significantly inhibited the growth of microorganisms in shrimp, delayed protein decomposition, and effectively maintained the sensory quality of shrimp. The shelf life of shrimp in the experimental group was extended to 7 days (while still being sensorily acceptable), while the control group spoiled on the 5th day, with a shelf life of less than 5 days. This fully demonstrates the good application potential of the composite film of this invention in actual food preservation.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite film, characterized by, Its raw materials include: modified nanocellulose and polyphenol extract in a mass ratio of 2-18:1; the modified nanocellulose is obtained by oxidative modification and nano-sizing of cellulose, and the carboxyl content on its surface is 0.5-1.8 mmol / g.

2. The composite film according to claim 1, characterized by, The steps for oxidative modification of the cellulose include: placing the cellulose in an oxidation system, reacting it for 1-2 h at a pH of 6-8 and a temperature of 0-10 ℃, then adjusting the pH to 9.5-10.5, and raising the temperature to 20-30 ℃ to continue the reaction for 3-8 h; Preferably, the oxidation system comprises TEMPO, NaBr, and NaClO in a mass ratio of 1:(5-10):(50-100), wherein the effective chlorine content of NaClO relative to the cellulose is 4-10 mmol / g; Preferably, the effective chlorine concentration in the NaClO is 10%-15% (w / w).

3. The composite film of claim 2, wherein, The cellulose is selected from one or more of softwood pulp cellulose, hardwood pulp cellulose, bamboo pulp cellulose, and bacterial cellulose.

4. The composite film according to any one of claims 1 to 3, characterized in that, The total phenol content in the polyphenol extract is not less than 50 mg GAE / g; Preferably, the polyphenol extract is obtained from one or more raw materials selected from rose petals, grape seeds, grape skins, and tea leaves.

5. The composite film according to any one of claims 1 to 3, characterized in that, The modified nanocellulose has a cross-sectional diameter of 5-40 nm and a length of 200-5000 nm. Preferably, the degree of polymerization of the modified nanocellulose is ≥200.

6. The composite film according to any one of claims 1 to 3, wherein The raw materials also include plasticizers and / or crosslinking agents, wherein the plasticizers constitute 0.5-5% by mass of the raw materials, and the crosslinking agents constitute 0.1-2% by mass of the raw materials. Preferably, the plasticizer includes one or more of glycerin, sorbitol, and polyethylene glycol; Preferably, the crosslinking agent includes one or more of glutaraldehyde and epichlorohydrin.

7. The composite film according to any one of claims 1 to 3, wherein The raw materials comprise the following components by weight percentage: 60-90% modified nanocellulose, 5-30% polyphenol extract, 0.5-5% plasticizer, and 0.1-2% crosslinking agent.

8. The composite membrane according to any one of claims 1-7, characterized in that, The composite membrane has at least one of the following characteristics: a. Transmittance at 600 nm wavelength ≥ 85%; b. UV blocking rate at 400 nm wavelength ≥95%; c. total phenol migration < 2.5 pg / dm in a simulated migration test at 40°C for 10 days 2 ; Preferably, the composite membrane also has at least one of the following characteristics: d and e: d. Thickness is 20-80 μm; e. Tensile strength ≥ 70 MPa.

9. The method for preparing the composite membrane according to any one of claims 1-8, characterized in that, Includes the following steps: The modified nanocellulose, the polyphenol extract, the plasticizer, and the crosslinking agent are mixed evenly and then subjected to ultrasonic treatment to obtain a film-forming solution. The film-forming solution is then cast onto a substrate to form a film, which is then dried. Preferably, the modified nanocellulose is provided in the form of a dispersion, wherein the mass of the modified nanocellulose accounts for 0.5-2% of the mass of the dispersion; Preferably, the polyphenol extract is provided in the form of a solution with a mass concentration of 0.5-5%.

10. The application of the composite film according to any one of claims 1-8 or the composite film prepared by the preparation method according to claim 9 in food packaging and food preservation; Preferably, the composite film is used in the preservation of fresh shrimp.

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